Last month, there were so few sockeye salmon in the Fraser River that all fishing was banned. But this week, with wave after wave of pink salmon arriving, the fishery was opened and processing plants were quickly overwhelmed by the huge numbers of salmon coming in.

The massive return of one species – pinks – coming on the heels of a disastrous run of another – sockeye – may be linked to a dramatic shift in ocean conditions last year. And it has raised questions about the possible role of a controversial experiment that took place when the Haida Salmon Restoration Corp. dumped iron material in the ocean last summer, stimulating plankton growth just as the pink salmon were moving through the area.

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Researchers say it is too soon to say if the experiment boosted salmon returns, but it is clear there are far more fish coming back now than anyone expected.

“Buyers begged the Sto:lo not to go fishing pinks this week because they can’t service our fishermen,” Ernie Crey, fisheries adviser to the Sto:lo Tribal Council, said in an e-mail. “The haul was so large that the fish plants didn’t have the capacity to handle the fish. Fish totes [or packing boxes] can’t be found and there is no ice to buy.”

The Pacific Salmon Commission, which had been anticipating a return of about 12 million pinks, now says an estimated 26 million fish are in the Fraser. Other rivers are experiencing remarkable returns as well.

“The fish poured in here in such numbers that the sea was a sparkling diamond,” said Ken Kirkby, an artist who has been working to restore salmon runs in Nile Creek on Vancouver Island. “You can’t see the bottom of the river for the salmon.”

Jeff Grout, Pacific region salmon manager for the federal Department of Fisheries and Oceans (DFO), said there are huge runs of pink salmon all along the coast from the Fraser in the south to the Nass River north of Prince Rupert.

Mr. Grout said run size estimates aren’t available for most rivers yet, but commercial catches have been extremely large. In a fishery near the Nass, the fleet took two million pink salmon; off the Kitimat River, the catch was five million. Off the mouth of the Fraser, 1.3 million pinks were caught in a brief opening this week. That fishery was cut short because of concerns about coho and sockeye that are also migrating, but more openings are expected soon.

One of the biggest surprises has been in the Squamish River, where so many fish have returned that commercial seine boats were allowed to set their nets near the mouth for the first time in decades. They caught 300,000 salmon in a few days.

Jonn Matsen, co-ordinator of the Squamish Streamkeepers, a non-profit group that for years has been working to restore salmon habitat, said the number of fish in the river is astonishing. “There’s a ridiculous number of pinks,” he said. “It’s a thousand times more than there should be.”

Dr. Matsen said he wondered if the Haida experiment, which saw 100 tonnes of iron sulphate dumped north of Haida Gwaii, could have caused the huge returns. The Haida experiment is under investigation by Environment Canada because ocean fertilization is not allowed under the Canadian Environmental Protection Act.

A DFO ocean scientist was not available to discuss the issue on Thursday, but the department did forward a research document which noted the Haida experiment had caused a bloom in August, 2012. It also stated there were natural plankton blooms off Vancouver Island and that ocean conditions in 2012 were cool, which is good for salmon.

Peter Gross, senior oceanographic engineer with the Haida Salmon Restoration Corp., said “we can’t prove at this time” that the experiment and the salmon boom are linked, “but it might be related.” He said more research is needed to understand the situation.

In 2008, a volcano showered the Gulf of Alaska with iron-rich ash – and two years later, a sockeye run that was in the area feeding at the time came back in record numbers.